An eruptive rock allied to trachyte, consisting essentially of a triclinic feldspar, with pyroxene, hornblende, or hypersthene.
The dark, aphanitic, extrusive rock that has a silica content of about 60% and is the second most abundant volcanic rock. Andesites are found in large quantities in the Andes Mountains.
A fine-grained igneous rock composed mostly of plagioclase feldspar and from 20% to 40% pyroxene, amphibole, or biotite, but no quartz or K-feldspar. It is abundant in mountains bordering the Pacific Ocean, such as the Andes Mountains of South America, from which the name was derived.